1. Field of the Invention
This invention relates to an optional display detecting apparatus used in a computer system allowing selective connection of various optional displays, for sensing which optional display is now connected.
2. Description of the Related Art
In recent years, computer systems have suddenly been popularized in personal use. This tendency is largely ascribed to the existence of laptop-type or book-type computers developed as a result of remarkable progress in computer technology. Those types of computers generally incorporate a liquid-crystal display or a plasma display, featuring thinness, light weight, and low-power consumption, as a standard specification. The improvement in the computer processing capability requires higher display resolution and more display colors. Unlike CRT displays, however, the aforementioned standard display cannot easily fulfill such requirements. For this reason, some computers are designed for allowing connection of the CRT display as an optional display.
The Basic Input/Output System (BIOS) of such computers includes an initialization and reliability test routine to be executed upon start of power supply. Part of this routine is programmed to perform an automatic display selecting process. In the display selecting process, the computer checks to see if an optional display is connected thereto. The optional display is selected when the connection is detected, and the standard display is selected when no connection is detected. For example, when a video Graphic Array (VGA) adapter is used for controlling the selected display, this VGA adapter reads out image data for one frame from a video RAM, produces a series of pixel video data corresponding to the image data, and supplies the series of pixel video data to a video Digital/Analog Converter (DAC). The video DAC has a number of color registers each storing color data constituted by color values of Red, Green, and Blue. Each pixel video data from the VGA adapter selects one of the color registers in the video DAC. The video DAC converts the R, G, and B values of the color data stored in the selected color register into analog voltages, and outputs them from RGB output terminals as analog RGB signals. In the display selecting process, particular test image data are stored in the video RAM so that predetermined analog RGB signals are continuously output from the RGB output terminals. Once a CRT display is connected to the RGB output terminals, the voltage levels at the RGB output terminals vary with the load resistance inherent to the CRT display. Therefore, voltage levels at the RGB output terminals are checked to determine the connection and type of the CRT display.
Generally, the laptop-type or book-type computer supports a resume function for resuming the operation conditions of system upon start of power supply. This resume function is made active when a resume mode is set in a set-up process for the computer. In the resume mode, the system data (i.e., the instruction data, processed data, status data, and the like, stored in the internal registers of a CPU) are saved in a backup memory before cutting off of power supply, and restored from the backup memory upon restart of power supply. Since the system data is not destroyed during the suspension of power supply, execution of a program can be continued from the point of interruption after the restart of power supply.
With this type of computer, however, the automatic display selecting process is not effected when the power switch is turned on in the resume mode. Therefore, if the optional display is removed during the suspension of power supply, no data can be displayed in either optional or standard display. To avoid this problem, an attempt could be made to check again to see if the optional display is connected. This, however, would create another problem. That is, the contents of the video RAM would be destroyed by the test image data prepared for the check. Therefore, it is necessary to secure an auxiliary memory for temporarily storing image data saved from the video RAM before storing the test image data. In addition, a lot of time is required to access the auxiliary memory in saving and restoring the contents of the video RAM. To avoid this problem, an attempt could be made to store identical test color data in all the color registers of the video DAC, instead of storing image data in the video RAM. However, the contents of the color registers would be destroyed by the test color data prepared for the check. Therefore, it is necessary to secure an auxiliary memory for temporarily storing color data saved from the color registers before storing the test color data. In addition, a lot of time is required to access the auxiliary memory in saving and restoring the contents of the color registers.